Of course I can't reproduce it now! However, about 20% of the time I try to login to my Gmail account (because my wife is logged into her), it gets stuck in an infinite redirect loop. I always end up surfing around to different Google sites for a while, and then eventually it fixes itself.

I found that it was the Smart Labels feature available in Labs that caused the infinite redirect loop. After using http://mail.google.com/mail/?labs=0 to get into my mail, I disabled that lab feature. After that, I could access email again normally. It's a shame this answer (specifically to use the labs=0 URL) isn't listed anywhere in the Google Apps help site.

I have multiple accounts that I sign in and out of. The redirect loop is not normal. Just to clarify, I was not saying clearing data needed to be done every time, just once to get out of the corrupted state.
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Rebecca ChernoffJul 18 '10 at 2:51